I would suggest going to San Antonio and getting the Sea World play passes for 2 of you and one AP for the other. (Then you can use the perks of the AP like free parking.)
We had a great Spring Break just visiting Sea World, Natural Bridge Caverns, The Alamo and Riverwalk. We always do the riverboat tour once every trip. We have visited Fiesta Texas before. If you are interested in that, definitely get the Six Flags passes so that you can use them in Arlington when you get home too.
We have found that we prefer to stay OFF the Riverwalk as the cost is not really worth it for our family once you add in parking fees.
Some suggested Fossil Rim and that is fun, but if you are going in the summer, I wouldn't do that one. It gets hot driving though the park with your windows down, even if you keep the AC on. There are some other things to do in the Glen Rose area but to me that is more of a day trip or overnight trip than a vacation. Just my opinion!
We looked at going to Branson MO. for Spring Break but many of the shows were closed because it was the off season for them. It looked like there were several hotels with waterparks. (Not like Great Wolf but still cool!) I think that we decided it was about 10 hours for us to drive. It might be a little less for you, depending where you are in Dallas.